A multilayer printed wiring board is known as a technology to permit high-density packaging and the shortest connection of components. The term “connection” as used herein means electrical continuity. An interstitial via hole (IVH) technique is applied to the manufacture of a multilayer printed wiring board in which higher-density packaging is required. The IVH technique is characterized in that a via hole passing through adjacent layers is filled with a conductive material so as to connect the adjacent layers. According to the IVH technique, the interlayer connection can be formed only at a portion where the connection is required. In addition, a component can be mounted on the via hole. Hence, the IVH technique enables high density wiring allowing a high degree of flexibility.
For example, Fujikura Technical Review 107, pp. 37-41 (October, 2004) describes a multilayer printed wiring board manufactured by stacking single-side copper clad polyimide substrates by means of the IVH technique. FIG. 4 is a process drawing illustrating the process of manufacturing the multilayer printed wiring board.
First, a copper foil of a single-side copper clad polyimide substrate (CCL: FIG. 4a) is etched to form a circuit (FIG. 4b). After an adhesive sheet 23 is laminated on the single-side copper clad polyimide substrate (FIG. 4c), via holes are formed by a perforation process utilizing laser irradiation (FIG. 4d). After the via holes are filled with a conductive paste by screen printing, a substrate 26′ that is manufactured by the same method and a single-side copper clad substrate 27 including a circuit are stacked on the adhesive sheet 23 while adjusting their positions. Their interply adhesion is performed altogether by heating under pressure so that a multilayer printed wiring board is formed.
For the purpose of filling such a via hole, a conductive paste which is made by dispersing conductive fillers such as metal powders in a resin binder is widely used. For example, Patent document 1 discloses a conductive paste composition used for filling a via hole, which composition is made of a liquid epoxy resin in which a plurality of electroconductive powders having different average particle diameters are dispersed. According to the description of Patent document 1, the fill ration of the electroconductive powders in a via hole can be increased by blending the electroconductive powders having different average particle diameters, and consequently the reliability of connection can be improved.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-92024.
Non-patent Document 1: Fujikura Technical Review 107, pp. 37-41 (October, 2004)